You start the interactive shell with erl and quit with Ctrl-g q. Read about Erlang here [http://learnyousomeerlang.com]

== Introduction ==

== Introduction ==

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Erlang is a programming language which has many features more commonly associated with an operating system than with a programming language: concurrent processes, scheduling, memory management, distribution, networking, etc. On this page I will show how to use Erlang on RPi to put a sensor value from Gertboard on the web using an interface written in [[Forth]].

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On this page I will show how to use Erlang on RPi to put a sensor value from Gertboard on the web using an interface written in [[Forth]] and how to put a picture from Pi Camera on the web.

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== Concurrency ==

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Erlang has extremely lightweight processes whose memory requirements can vary dynamically. Processes have no shared memory and communicate by asynchronous message passing. Erlang supports applications with very large numbers of concurrent processes. No requirements for concurrency are placed on the host operating system.

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== Distribution ==

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Erlang is designed to be run in a distributed environment. An Erlang virtual machine is called an Erlang node. A distributed Erlang system is a network of Erlang nodes (typically one per processor). An Erlang node can create parallel processes running on other nodes, which perhaps use other operating systems. Processes residing on different nodes communicate in exactly the same was as processes residing on the same node.

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== Robustness ==

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Erlang has various error detection primitives which can be used to structure fault-tolerant systems. For example, processes can monitor the status and activities of other processes, even if these processes are executing on other nodes. Processes in a distributed system can be configured to fail-over to other nodes in case of failures and automatically migrate back to recovered nodes.

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== Soft real-time ==

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Erlang supports programming "soft" real-time systems, which require response times in the order of milliseconds. Long garbage collection delays in such systems are unacceptable, so Erlang uses incremental garbage collection techniques.

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== Hot code loading ==

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Many systems cannot be stopped for software maintenance. Erlang allows program code to be changed in a running system. Old code can be phased out and replaced by new code. During the transition, both old code and new code can coexist. It is thus possible to install bug fixes and upgrades in a running system without disturbing its operation.

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== Incremental code loading ==

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Users can control in detail how code is loaded. In embedded systems, all code is usually loaded at boot time. In development systems, code is loaded when it is needed, even when the system is running. If testing uncovers bugs, only the buggy code need be replaced.

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== A Web Server ==

== A Web Server ==

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Erlang is a perfect language to use for programming a web server, and that is exactly what Loïc Hoguin did. Cowboy is one of the most scalable web servers there is and it's also small so it fits nicely in a RPi. I was thinking of using it to display the kW load from my [[Forth | Atlast Forth]] interface to [[RPi Gertboard | Gertboard]]. Here is how I did it, first install the cowboy web server and test run it:

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Erlang is a perfect language to use for programming a web server, and that is exactly what Loïc Hoguin did. [http://www.ninenines.eu] Cowboy is one of the most scalable web servers there is and it's also small so it fits nicely in a RPi. I was thinking of using it to display the kW load from my [[Forth | Atlast Forth]] interface to [[RPi Gertboard | Gertboard]]. Here is how I did it, first install the cowboy web server and test run it:

<code><pre>

<code><pre>

sudo apt-get install git

sudo apt-get install git

Line 45:

Line 27:

To make it run you first have to tell it to accept HTTP:

To make it run you first have to tell it to accept HTTP:

<code><pre>

<code><pre>

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cd ebin

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cd ~/cowboy/ebin

cp examples/hello_world/src/hello_world.app.src ./hello_world.app

cp examples/hello_world/src/hello_world.app.src ./hello_world.app

erlc ../examples/hello_world/src/*.erl

erlc ../examples/hello_world/src/*.erl

cd ..

cd ..

cp examples/hello_world/start.sh .

cp examples/hello_world/start.sh .

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sudo ./start.sh

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./start.sh

</pre></code>

</pre></code>

Start a browser and point it to your RPi:8080, eg 192.168.0.178:8080

Start a browser and point it to your RPi:8080, eg 192.168.0.178:8080

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== External interfaces ==

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== External interfaces - Gertboard ==

Erlang processes communicate with the outside world using the same message passing mechanism as used between Erlang processes. This mechanism is used for communication with the host operating system and for interaction with programs written in other languages. If required for reasons of efficiency, a special version of this concept allows e.g. C programs to be directly linked into the Erlang runtime system. The easy way is good enough so just open a port to [[Forth | Atlast Forth]] in the file examples/hello_world/src/toppage_handler.erl. Replace toppage_handler.erl with the following:

Erlang processes communicate with the outside world using the same message passing mechanism as used between Erlang processes. This mechanism is used for communication with the host operating system and for interaction with programs written in other languages. If required for reasons of efficiency, a special version of this concept allows e.g. C programs to be directly linked into the Erlang runtime system. The easy way is good enough so just open a port to [[Forth | Atlast Forth]] in the file examples/hello_world/src/toppage_handler.erl. Replace toppage_handler.erl with the following:

<code><pre>

<code><pre>

Line 85:

Line 67:

cd to ebin and compile the file:

cd to ebin and compile the file:

<code><pre>

<code><pre>

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cd ebin

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cd ~/cowboy/ebin

erlc ../examples/hello_world/src/toppage_handler.erl

erlc ../examples/hello_world/src/toppage_handler.erl

</pre></code>

</pre></code>

Line 94:

Line 76:

In directory cowboy, create a directory priv and copy atlast to it:

In directory cowboy, create a directory priv and copy atlast to it:

<code><pre>

<code><pre>

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cd ~/cowboy

mkdir priv

mkdir priv

cp ../atlast-1.2/atlast ./priv

cp ../atlast-1.2/atlast ./priv

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Line 89:

: w 25 getkwh 2drop 25 getkwh 36 * swap 100 / / ." "w=" . cr ;

: w 25 getkwh 2drop 25 getkwh 36 * swap 100 / / ." "w=" . cr ;

</pre></code>

</pre></code>

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Ok, now we just have to call start.sh again and point the browser to 192.168.0.178:8080 or whatever ip address you have on your RPi. After a few seconds the W load should show up. I stop here, this is not the right forum to discuss how to make a web gui, lots of other sites can help with that. Have fun :)

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Ok, now we just have to call start.sh again ( sudo is needed by the Gertboard driver, not by Cowboy, and for pure simplicity I take another shortcut and starts the Cowboy with sudo rights. Don't do this in a production environment).

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<code><pre>

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sudo ./start.sh

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</pre></code>

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Point the browser to 192.168.0.178:8080 or whatever ip address you have on your RPi. After a few seconds the W load should show up. I stop here, this is not the right forum to discuss how to make a web gui, lots of other sites can help with that. Have fun :)

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Note: The suggested way to call the port directly from examples/hello_world/src/toppage_handler.erl is just to make it easy. In a real website with thousands of users you should open the port from a separate process and let it run and serve more than one web call.

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== External Interfaces - Pi Camera ==

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If you have a PiCamera and wants to have a picture on a web page. The cool thing with this is that it's in real time, it is not a file that is served but a new picture is taken when you do the web call. Check that /opt/vc/bin/raspistill works on your system, then replace the code in examples/hello_world/src/toppage_handler.erl with the following: (Then follow the Gertboard example for compiling and running).

Note: The suggested way to call the port directly from examples/hello_world/src/toppage_handler.erl is just to make it easy. In a real website with thousands of users you should open the port from a separate process and let it run and serve more than one web call.

Note: The suggested way to call the port directly from examples/hello_world/src/toppage_handler.erl is just to make it easy. In a real website with thousands of users you should open the port from a separate process and let it run and serve more than one web call.

[[Category: RaspberryPi]]

[[Category: RaspberryPi]]

Revision as of 20:56, 27 July 2013

Erlang on RPi, starting with a fresh 2013-05-25-wheezy-raspbian.zip image.

Contents

Introduction

On this page I will show how to use Erlang on RPi to put a sensor value from Gertboard on the web using an interface written in Forth and how to put a picture from Pi Camera on the web.

A Web Server

Erlang is a perfect language to use for programming a web server, and that is exactly what Loïc Hoguin did. [2] Cowboy is one of the most scalable web servers there is and it's also small so it fits nicely in a RPi. I was thinking of using it to display the kW load from my Atlast Forth interface to Gertboard. Here is how I did it, first install the cowboy web server and test run it:

External interfaces - Gertboard

Erlang processes communicate with the outside world using the same message passing mechanism as used between Erlang processes. This mechanism is used for communication with the host operating system and for interaction with programs written in other languages. If required for reasons of efficiency, a special version of this concept allows e.g. C programs to be directly linked into the Erlang runtime system. The easy way is good enough so just open a port to Atlast Forth in the file examples/hello_world/src/toppage_handler.erl. Replace toppage_handler.erl with the following:

Ok, now we just have to call start.sh again ( sudo is needed by the Gertboard driver, not by Cowboy, and for pure simplicity I take another shortcut and starts the Cowboy with sudo rights. Don't do this in a production environment).

sudo ./start.sh

Point the browser to 192.168.0.178:8080 or whatever ip address you have on your RPi. After a few seconds the W load should show up. I stop here, this is not the right forum to discuss how to make a web gui, lots of other sites can help with that. Have fun :)

Note: The suggested way to call the port directly from examples/hello_world/src/toppage_handler.erl is just to make it easy. In a real website with thousands of users you should open the port from a separate process and let it run and serve more than one web call.

External Interfaces - Pi Camera

If you have a PiCamera and wants to have a picture on a web page. The cool thing with this is that it's in real time, it is not a file that is served but a new picture is taken when you do the web call. Check that /opt/vc/bin/raspistill works on your system, then replace the code in examples/hello_world/src/toppage_handler.erl with the following: (Then follow the Gertboard example for compiling and running).

Note: The suggested way to call the port directly from examples/hello_world/src/toppage_handler.erl is just to make it easy. In a real website with thousands of users you should open the port from a separate process and let it run and serve more than one web call.